As a pressure type flowmeter, there is one adapted to include a fluid resistance part in a flow path through which fluid flows, include pressure sensors respectively on the upstream and downstream sides of the fluid resistance part, and on the basis of the differential pressure between the upstream side pressure sensor and the downstream side pressure sensor, measure the flow rate of the fluid.
As the upstream and downstream side pressure sensors, capacitance type pressure sensors having the same configuration are respectively used. Each of the capacitance type pressure sensors is one adapted to measure absolute pressure by detecting a change in the capacitance between a diaphragm displaceable by pressure and a fixed electrode.
Specifically, the capacitance type pressure sensor includes: a main body part joined with the diaphragm; and the fixed electrode that is inserted into an electrode fixing hole formed in the main body part and provided with the fore end surface thereof facing the diaphragm, and includes a cylindrically-shaped guard electrode between the fixed electrode and the main body part to suppress floating capacitance from occurring between the fixed electrode and the main body part. In addition, the electrode fixing hole is provided with: a first sealing glass that insulates the fixed electrode and the guard electrode from each other as well as connecting them to each other; and a second sealing glass that insulates the guard electrode and the main body part from each other as well as connecting them to each other.
However, a gap is likely to occur between the guard electrode and the sealing glasses, and internal gas enclosed in an airtight reference chamber formed between the diaphragm and the fixed electrode may gradually leak through the gap to consequently prevent accurate pressure measurement. In the pressure type flowmeter adapted to measure a flow rate on the basis of the differential pressure between upstream side pressure and downstream side pressure, a change in the state of the airtight reference chamber of the upstream side pressure sensor and/or a change in the state of the airtight reference chamber of the downstream side pressure sensor cause a flow rate measurement error.